1. Field of the Invention
The present invention relates to a porous membrane which is used for a microfiltration membrane for drugs or bacteria, or which is used for a separator of a battery. More particularly, the invention relates to a vinylidene fluoride porous membrane which has an asymmetrical structure in which pores in opposite sides have different average diameters, which has excellent mechanical strength such as tensile strength at break and elongation at break, and which has pores having a diameter falling in a narrow range.
2. Description of Related Art
Porous membranes made of synthetic resins have conventionally found utility in many technical fields. For example, they are used for separating gases from gases, gases from liquids, and solids from liquids. They are also used as electric insulators, heat reserving materials, sound insulators, and heat insulators. When a porous membrane is used for separation, it must meet the following requirements that affect its performance. First, the porous membrane must have a suitable porosity in view of the separation efficiency. Second, the diameter of the pores must be uniform to achieve precise separation. Third, the membrane must have pores having a diameter which is suitable for the object to be separated. Materials which constitute a membrane are required to have chemical resistance to the object to be separated, wheatherability, heat resistance, and strength. Moreover, porous membranes must have sufficient levels of elongation at break and tensile strength at break. Presently, vinylidene fluoride resins, which have excellent wheatherability, chemical resistance, heat resistance, and mechanical strength, have been studied in hopes of employing them as porous membranes for separation.
Vinylidene fluoride resins, however, do not necessarily have desirable processing properties due to their non-adhesive character and poor compatibility. In addition, development of porous membranes has been focused on enhanced porosity and a narrow range distribution of the pore diameters, and so development of mechanical strength has not been emphasized. Accordingly, when a porous membrane is used as a filter membrane, a supporting membrane is superposed on the porous membrane in order to increase the mechanical strength. When a porous membrane is used as a separator of batteries, it desirably has a mechanical strength (elongation at break, tensile strength at break) that sufficiently withstands the winding forces incurred as it is wound around a core material in the manufacture of batteries. In addition, when used as a separator of batteries, a porous membrane desirably has penetrating pores having a diameter falling within a narrow range so that fine powders of the active material used in an electrode cannot pass therethrough, and it also has impregnating properties that allow an electrolyte to be impregnated efficiently in the membrane after it is wound on the core material. When a porous membrane is used as a microfiltration membrane, it is required to maintain high filtering properties for a prolonged period of time.
Japanese Patent Application Laid-open (kokai) No. 215535/1991 discloses a method in which an organic liquid such as diethyl phthalate and an organic fine powder (hydrophobic silica) are added to a polyvinylidene fluoride resin, and after the obtained mixture is melt-processed, the organic liquid and the hydrophobic silica are extracted. This method provides a porous membrane having relatively high mechanical strength. However, in the porous membrane according to this method, the average diameter of the pores in opposite sides of the membrane are not significantly different from each other, and are in fact almost identical. Thus, the membrane of that publication does not have an asymmetrical structure. Consequently, highly efficient impregnating properties desired of a battery separator, or long-term filtering properties desired of a microfiltration membrane cannot be expected. Moreover, an aqueous alkaline solution employed for extracting hydrophobic silica tends to cause deterioration of vinylidene fluoride resin.
In view of the foregoing, the inventors of the present invention conducted extensive studies and found that a vinylidene fluoride porous membrane of an asymmetrical structure having a highly efficient impregnating property when used as a separator for a battery and long-lasting high filtration properties when used as a microfiltration membrane, having excellent mechanical strength such as elongation at break or tensile strength at break, can be obtained by a method in which a vinylidene fluoride resin having a specific inherent viscosity and a specific plasticizer are formed into a membrane by melt-processing, followed by a specific post-treatment.